@Pete ******
you don't need an agent it is very easy.
You need to apply for a non O visa online at the Thai Embassy in your country before coming to Thailand.
Enter Thailand using the visa.
Open a Thai bank account immediately and deposit the 800,000.
Go to immigration and ask for a list of requirements to extend your stay based on retirement.
Wait until your money has seasoned for 2 months, then go to immigration with everything on their list and pay 1,900 baht.
The form to apply is a 2 sided A4 form with about 15 questions, things like; name, passport number, date of arrival in Thailand etc.
Good luck.